Plasmodium spp. are parasitic protozoa and the human infection caused by these organisms is known as malaria. Malaria is a devastating infectious disease. There are over 300 million cases per year worldwide and resulting in approximately 660,000 million deaths per year. Plasmodium species that infect humans are transmitted by the bite of an infected Anopheline mosquito. Plasmodium falciparum is responsible for most of the deaths due to malaria. However, Plasmodium vivax is the most prevalent species worldwide and causes a significant amount of morbidity. Plasmodium falciparum, the cause of the most virulent form of malaria, has developed resistance to currently used drugs. This in turn has led to an increase in the incidence of malaria and to fewer drugs for both treatment and prophylaxis of the disease. Other species of Plasmodium, such as P. knowlesi and P. ovale, although they principally infect other animals, may also cause disease in humans.
Malaria infection is initiated when an infected Anopheline mosquito injects sporozoites into a subject during the mosquito's blood meal. After injection, the parasite enters the bloodstream and undergoes a series of changes as part of its lifecycle. The sporozoite travels to the liver where it invades hepatocytes. One sporozoite can generate over 10,000 merozoites which will then rupture from the hepatocyte and invade erythrocytes, although P. vivax and P. ovale can remain dormant in the liver (hypnozoites) and cause relapses years after the initial infection. In their intraerythrocyte phase, the merozoites go through various forms (rings, trophozoites, schizonts) to form an average of 20 daughter merozoites that are released into the bloodstream and infect new red blood cells (Biamonte et al., 2013). This causes the symptomatic high fevers and associated pathology.
The disease malaria is usually confirmed by the microscopic examination of blood films or by antigen-based rapid diagnostic tests (RDT). Microscopy is the most commonly used method to detect the malarial parasite—about 165 million blood films were examined for malaria in 2010 (Aregawi et al., 2011). Despite its widespread usage, diagnosis by microscopy suffers from two main drawbacks: many settings (especially rural) are not equipped to perform the test, and the accuracy of the results depends on both the skill of the person examining the blood film and the levels of the parasite in the blood. The sensitivity of blood films ranges from 75-90% in optimum conditions, to as low as 50%. Commercially available RDTs may be more accurate than blood films at predicting the presence of malaria parasites, but they are widely variable in diagnostic sensitivity and specificity depending on manufacturer, and are unable to tell how many parasites are present (Wilson, 2012).
Antimalarial drug resistance has emerged as one of the greatest challenges facing malaria control today. Drug resistance has been implicated in the spread of malaria to new areas and re-emergence of malaria in areas where the disease had been eradicated. Drug resistance has also played a significant role in the occurrence and severity of epidemics in some parts of the world (Bloland, 2001). Unfortunately, current techniques to detect resistance, such as PCR analysis, require several weeks to be completed.
There is a need to provide faster, cheaper and non-invasive ways to detect Plasmodium infection in particular low levels of infection presented in children in pregnant women and to monitor the effectiveness of treatment, especially in the face of drug-resistant strains of Plasmodium spp. There is also the need for new ways of screening for compounds to treat Plasmodium infection.